This invention relates to an electrostatic dust collector and, more particularly, to an electrostatic dust collector in which extremely small particles of dust can be collected efficiently with an easily replaceable filter, and wherein a short-circuit discharge caused by the application of a high voltage does not occur.
The worsening of municipal air pollution caused by the growth of industry and the overcrowding of cities is a hindrance to productivity in the industrial sector and has a deleterious effect in terms of environment and health in residential areas. Accordingly, pollution preventing measures such as controlling the source of pollutants have been studied and put into effect. The purification of air in limited spaces is also an important consideration in many sophisticated and diverse fields. In particular, air purifiers which includes means for dealing with dust, smoke (especially tobacco smoke), ticks and pollen are essential to raise the yield at which such products as VLSIs (very large-scale integrated circuits) of a very high precision are produced through ultra-fine, precise machining in the semiconductor industry. They are also absolutely necessary in operating and aseptic rooms, in bacteriological experiments, for furthering biotechnological research in food processing, and for improving the environmental hygiene in the home, working place and recreational facilities. The high-performance filters required are steadily being improved to deal with free-floating particles having a diameter of at least 0.3 micron, and is some cases 0.1 micron. Good results are gradually being obtained.
The dust collecting mechanisms employed in conventional air purifiers are classified roughly as being of the mechanical dust collecting or electrical dust collecting type, depending upon the operating principle. Generally speaking, the mechanical dust collecting systems are capable of trapping particles of a large diameter only and involve many difficulties in terms of installation and handling. These days the electrical dust collectors are the most commonly employed.
Electrical dust collecting systems include electrostatic dust collectors in which dust is trapped electrostatically upon being ionized by a corona discharge, and electrostatic induction-type air purifiers in which an electric field is applied across an inductor and dust is passed through the inductor to be trapped electrostatically. Let us first describe a conventional example of the former, namely the electrostatic dust collector, with reference to FIGS. 1 and 2.
FIG. 1 is useful in describing the dust collecting principle of the electrostatic dust collector. Floating particles contained in polluted air 1 pass through a filter 2 and are positively charged in a charging section 3 having a discharging wire 4 for effecting a corona discharge. The positively charged particles enter a collecting section 5 where they are repelled by high-voltage electrode plates 6 and trapped by grounded electrode plates 7. The apparatus thus provides purified air 8 from which the floating particles have been removed.
FIG. 2 is a sectional view illustrating an example of an electrostatic dust collector that employs the foregoing dust collecting principle. The dust collector includes a unit in which are assembled a discharge wire 10 and a discharge electrode plate 11, both having a positive potential, and a dust collecting electrode plate 12 having a negative potential. The unit is contained in a holder section 13 having a front side in which a front filter 14 is set, and a rear side in which a rear filter 15 and an activated carbon filter 16 for odor removal are installed. The unit with the attached filters is installed in a casing 17 through an intake port covered by a grill 18. A fan 19 and an outflow port 20 for the exiting air are provided in the rear portion of the casing 17.
Floating particles contained in polluted air are drawn in from the intake port by the fan 19, pass through the front filter 14 and are positively charged by the corona discharge wire 10. The positively charged particles are repelled by the discharge electrode plate 11, the potential whereof is positive, and are trapped by the dust collecting electrode plate 12, whose potential is negative. A stream of air so purified is blown out of the outflow port 20 upon passing through the rear filter 15 and activated carbon filter 16.
The latter air purifier of electrostatic induction type has already been disclosed in the specification of Japanese Patent Application Laid-Open No. 59-19564, filed by the inventor whose invention is described in the present application. This air purifier will now be discussed in detail with reference to FIGS. 3, 4, 5(a) and 5(b).
Let us first describe the dust collecting principle with reference to FIG. 3. The electrostatic induction-type air purifier includes an air-permeable, porous inductor 30 on which opposing electrodes 31, 32 are disposed and across which a high DC voltage is impressed to produce a strong electric field in the inductor 30, thereby trapping floating particles which attempt to pass through the pores in the inductor.
In terms of structure, the air purifier includes a filter element 41 arranged in the center of a case 40. Air containing pollutant particles is drawn into the case 40 from an inflow port 43 by a fan 42. To prevent the filter element 41 from becoming clogged, a filter bag 44 is disposed within the case 40 for trapping coarser dust particles. As shown in FIG. 5(a), the filter element 41 includes a filter member obtained by providing a thin film 48 of a metal such as aluminum comprising a first electrode on one side surface of a porous induction member 47 made of urethane foam or the like, and forming a metallic thin film 49 as a second electrode so that the induction member 47 is embraced by the electrodes. As shown in FIG. 5(b), a plurality of these filter members are wound into a cylindrical shape and a high voltage from a DC high-voltage power supply 45 (FIG. 4) is applied across the adjacent electrodes 48, 49 via terminals 48a, 49b. Numeral 47a denotes a screen for supporting the filter element 41.
In operation, floating particles drawn in from the intake port 43 are physically trapped in the air-permeable pores of the filter members. At the same time, a strong electrostatic field is generated by the inductors arranged between the positive and negative electrodes, thereby charging the floating particles. The particles so charged are trapped in the walls of the pores constituting the porous inductors.
The conventional electrostatic dust collector shown in FIGS. 1 and 2 has a number of drawbacks, which will now be set forth.
(1) Cleaning and maintenance are difficult.
Since the dust collecting effect diminishes when a large quantity of dust becomes attached to the dust collecting plates, a cleaning solution is prepared by dissolving a weakly alkaline cleaning agent in warm water at a temperature of about 60.degree. C. The dust collecting unit is extracted from the opening of the grill 18 and the electrostatic collecting section, from which the front filter 14, rear filter 15 and activated carbon filter 16 have been detached, is immersed in the cleaning solution, usually for a period of about three hours, depending on the extent of contamination. The electrostatic collecting section is then shaken back and forth and from side to side while still immersed in the solution in order dislodge the contaminants. This must be done without touching the fine discharge wires 10. Any deposits on the dust collecting electrode plates 12 from smoke such as tobacco smoke are difficult to remove. If a brush or the like is used, care must be taken not to scrape the collecting plates.
(2) The trapped particles tend to re-scatter.
To trap particles with greater efficiency, either the applied voltage is raised or the portions to which the voltage is applied are increased in length. In either case, however, the trapped particles are re-scattered by a discharge which occurs due to concentration of the electric field at portions where the accumulated dust forms raised deposits on the collecting electrode plates.
(3) There is a tendency to produce radio wave interference.
When the corona discharge is generated, a high-frequency current flows into the ionized space, thus causing noisy radio reception.
(4) Ozone is produced.
The corona discharge is accompanied by the production of ozone, which can irritate or cause damage to mucous membranes.
The electrostatic induction-type air purifier illustrated in FIGS. 3, 5(a) and 5(b) also has a number of disadvantages.
(1) The purifier is uneconomical since the filter element is discarded with the strip-like electrode attached thereto when no longer usable.
(2) The apparatus cannot be made compact in size.
(3) The apparatus cannot be improved to withstand use in environments where the temperature and humidity are high.